1. The Field of the Invention
The invention relates to a method for eliminating the influence from the double layer capacitance charging on an electrical current measured using an electrochemical sensor or an electrode arrangement comprising one or more electrodes and a method for measurements of oxygen in blood in which method this elimination of the double layer is applied. The invention also relates to an apparatus for electrochemical measurements having means for eliminating the influence of the double-layer capacitance in measurements of concentrations and to the use of the apparatus according to the invention in conjunction with a pacemaker.
2. Description of the Prior Art
An electrochemical sensor normally comprises a working electrode (WE), a counter electrode (CE) and a reference electrode (RE)
In the description the following expressions are intended to have the following meaning:
Measuring potential: the applied potential, as related to a reference potential, during the measurement, in the description noted .PHI.. PA1 Floating potential: The potential, as related to a reference potential, an electrode will acquire when placed in an electrolyte and no current is allowed to pass through an outer circuit, i.e. not passing through the electrolyte, in the description denoted E.sub.0. PA1 at the working electrode 2 H.sub.2 O+O.sub.2 +4 e.sup.-.fwdarw.4 OH PA1 at the counter electrode 4 OH.sup.- +C.fwdarw.CO.sub.2 +2 H.sub.2 O+4 e.sup.-
It should be understood that these potentials actually refer to an arbitrarily chosen level, e.g. a common ground.
In the field of measuring electrical potentials in solutions one encounters the problem of assessing the influence of the double layer capacitance existing over the interface between the working electrode or sensor and the liquid phase.
There is, in general, a potential gradient across any interface separating two phases, e.g. a solid and a liquid phase, due to material constrants.
The potential difference across an interface may conveniently be pictured as an electrical double layer. One phase acquires a net negative charge (excess of electrons), and the other acquires a net positive charge (deficiency of electrons). Double layers exist not only at plane surfaces, but also surround solid particles suspended in a liquid medium. This double layer may be considered to be the equivalent of a simple parallel-plate capacitor. This phenomenon of course exists, e.g. at every electrode surface which is involved in any electro-chemical measurements.
Double layers of this kind are for instance discussed in Derek Pletcher, "A first course in Electrode Processes, pp 77-106, The Electrochemical Consultancy (Romsey) Ltd.(1991). The existence of the double layer and its influences on measurements made has two different aspects in Pacemaker Technology.
The first aspect is that a large influence from the double layer capacitance is sometimes a desired effect as is described in U.S. Pat. No. 4,602,637. A layer having a high double layer capacity, is provided at the phase boundary electrode/body fluids to minimize polarization rise during stimulation by roughening of the surface of the passive electrode (counter electrode). This is also the object of the invention described in U.S. Pat. No. 4,611,604.
The second aspect is that in measurements in which more than one measurement electrical potential is used as related to a common value, e.g. a common ground it would be of interest to eliminate the influence of the double layer capacitance, since the influence of the double layer capacitance varies with the applied measurement potential and depends on concentration variations in the liquid of the entities, e.g. ions, dissolved gas etc. to be measured.
The method and the apparatus will be discussed and described with reference to measurements of oxygen dissolved in blood, but may be adapted to other measurements of the same type.
The measurements using electrochemical methods make use of the fact that the oxygen molecules dissolved in the blood are chemically reduced on the surface of the working electrode (WE) when the potential during a measurement pulse is forced to the negative potential (about 1 volt) relative to a reference electrode/potential. The counter electrode (CE) is herebelow assumed, at least partly, to have a surface made from carbon. In the reduction process, hydroxide ions are produced and the amount of these ions is dependent on the concentration of dissolved oxygen according to the reactions:
The above is simplified description of what is happening in the liquid. Other reactions may also take place.
The electrical current flowing to the working electrode WE during the measurement pulses is carried by the hydroxide ions. This current, called the oxygen current (I.sub.pO2) is proportional to the amount of hydroxide ions formed on the surface of the working electrode (WE). During the measurement pulse the carbon coating of the counter British Specification 2 059 597 a method and an apparatus for determination of oxygen partial pressure measurements is described. The apparatus comprises an electrochemical cell having electrodes, in which successive electrical square wave polarizing voltage pulses are applied across the electrodes of the cell for measuring of the content of molecular oxygen diffusing through a membrane into the cell. The pulses results in currents through the cell having different waveforms during charging and discharging. It is recognized that there is an electrode drift with time. Thus a correction factor to be applied in the measurements has to be used. The correction factor is calculated using the integrated value of one charge waveform and a succeeding discharge waveform.
PCT Application WO 83/01687 concerns a method and apparatus for pulsed electroanalyses of an electroactive material in an electrolyte by using a cell and applying pulses of an On-potential and an Off-potential. The On- potential is chosen to give a Faradaic current and the Off-potentials is chosen such that no Faradaic current is present. The current is integrated for the time the On-potential is applied and at least part of the time when the Off-potential is applied and the integral is used for concentration measurements. An object of the invention is to eliminate the effect of the double layer formed.
Electrochemical determination of the oxygen concentration, particularly in biological matter, is also the subject of U.S. Pat. No. 4,853,091. A method and an apparatus for measurements over long periods of time is described in which use is made of three electrodes, a working electrode (WE), a reference electrode and a counter electrode (CE). Two potentials are cyclically applied to the working electrode with one potential (measuring potential) in the range -1,4 V.ltoreq..rho.Ag/AgCl.ltoreq.-0,4 V and the other potential, the recovery potential in the range -0,2 V.ltoreq..rho.Ag/AgCl.ltoreq.+0,2 V. At the recovery potential, there will be no oxygen current depending on the fact that the chosen potential will not be of a magnitude as to allow a reduction of oxygen at the working electrode.
However, there will still be a double layer forming on the electrode/s, which is dependent on the applied potential (conf. above), but this double layer will not exhibit the same capacitance as the double layer present when the measuring potential is applied. This problem is according to the cited patent solved by starting the measurement period some time after the measurement potential is applied in order to avoid the current induced by the changes in the double layer resulting from the application of the measurement potential. The problem is that it is not possible to decide when the influence from the double layer ceases to exist.